Actually, the Eee is a 630Mhz Celeron, and you can easily overclock it to 900Mhz. Updating the BIOS, gives you another option in it, for "performance mode", which ups it to 900Mhz. Still lasts forever, and doesnt get hot. You may or may not know this, but do not get the Surf model, if you want more ram. There is no removable hatch over the ram on those models, just on the G models there are. Takes 1 min to change out ram, and it well worth it. I got 2gigs for $35 off of newegg.
But to your question, yes it can run CS:S, and GTA. If it does it well enough, is up to you. I have XP Pro SP3 on mine, havent tried any games really. Just got it for email, web, saving/editing pics, and watching movies/tv shows.
The most popular website about the Asus Eee, and the
subforum for XP and gaming on it.
Some pics of mine, comparing the size. I have the 4G.